GASKET COATING
This invention is concerned with a gasket coating. The invention is applicable to a gasket coating which enhances sealing by filling small cracks and fissures in the surface of a sheet of the gasket and/or of the member against which the gasket seals ("micro-sealing").
Gaskets are used for sealing between two members, eg the block and head of an internal combustion engine, and provide a seal around a passage which passes from one member to the other. Accordingly, a gasket has to be resilient in order to press against the members and provide a fluid-tight seal. The resilience can be provided by utilising a relatively thick layer of resilient material such as a resilient rubber-based material. The resilience can alternatively be provided by resilient beads running along lines where sealing is required, the beads being formed of rubber-based material and being mounted on a sheet of the gasket. Where the gasket comprises a sheet of metal, eg stainless steel, the resilience can be provided by embossing the sheet to form resilient ridges which provide the seal. Thus, when such a gasket is clamped between two members, the clamping force resiliently deforms the resilient portions of the gasket which, therefore, press against the members to form the seal.
In gaskets where the seal is provided by embossed metal ridges, it is found that the seal may be compromised because the ridges are unable to enter into small cracks and fissures in the members so that gases and liquids can escape past the ridges. In order to overcome this problem, it is known to apply a thin coating (typically less than
200 microns in thickness) of a sealing-enhancing coating to at least the crests of the ridges. The coating is designed to deform under clamping pressure to fill cracks and fissures. Known sealing-enhancing coatings are based on elastomers, such as nitrile rubbers and fluoro-rubbers , and are often applied in a solvent-borne form.
Because fluoro-elastomers , nitrile rubbers and silicone elastomers all degrade or become brittle at temperatures above 250°C, sealing-enhancing coatings for micro-sealing at temperatures above 250 °C have not, to the applicants' knowledge been based on these materials. For example, silicone elastomers start to decompose at 200- 250 °C and either disintegrate or become very hard.
It is an object of the present invention to provide a sealing-enhancing coating which is based on a silicone resin and which can be used at temperatures above 250 °C.
The invention provides a sealing-enhancing coating of a gasket or a portion of a gasket, the coating being based on a silicone resin, characterised in that the coating comprises a siliconate dispersed within said resin.
It is, surprisingly, found that a coating according to the invention remains effective after heating to a temperature exceeding 400 °C. It is surmised that the siliconate promotes a higher degree of cross-linking during curing of the coating leading to higher heat resistance. The coating is also resistant to the fluids associated with internal combustion engines, le oil, water, anti-freeze, and fuel. Siliconates are, of course, different to both silicates and sil cones (including siloxanes and silanes) . Since the 1950's, siliconates have been used for coating masonry to inhibit mould growth, prevent salt ingress and prevent frost damage. Alkyl siliconates are produced by the hydrolysis of alkyl chlorosilanes to produce a gel
which is alkyl silane triol . The siliconate is produced by dissolving the gel m an aqueous alkaline solution, eg potassium hydroxide. The siliconate clearly has to be soluble m order to distribute it in the coating.
The siliconate used m the invention is preferably an alkali metal or ammonium siliconate, for example potassium methyl siliconate (CAS No 16589-43-8). Other potassium, sodium or lithium siliconates are possible. Preferably, the proportion of siliconate to resin is between 1:25 and 1 : 5 by dry weight .
The silicone resin may be a polymeric alkoxyfunctional siloxane, for example a polymeric ethoxy functional siloxane.
It is surprisingly found that, m a significant number of applications, a coating according to the invention, after drying and in a semi-cured state, is flexible enough to survive an embossing process and also heat-resistant enough to still provide a functional coating after a tempering step (eg heating to 420 °C for one hour) . This makes it feasible to apply the coating to a planar steel sheet, then emboss the sheet to form the ridges, and then temper the steel.
A coating according to the invention may also be used on gasket sheets which are absorbent so that the coating may be impregnated into the material of the sheet to key the coating to the sheet.
It is a further object of the present invention to provide an improved method of forming a gasket having an improved sealing-enhancing coating.
The invention also provides a method of forming a gasket having a sealing-enhancing coating, the method
comprising forming a spreadable mixture comprising silicone resin and a siliconate solution, spreading the mixture over at least a portion of a sheet of a gasket, and curing the mixture.
In a method according to the invention, the mixture may be spread by any of the conventional methods such as screen printing, roller coating, etc.
In order to reduce the use of potentially harmful solvents, it is preferred that, in a method according to the present invention, the silicone resin is substantially free of organic solvents and water, ie the silicone resin is a high solids liquid eg a "100% solids liquid".
It is preferred that the mixture also contains a curing agent. The curing agent may be selected from one or more of polybutyltitanate and similar organotitanium complexes, organo-metallic tin complexes (eg dibutylin diacetate and dibutylin dilaurate) , organo-aluminium complexes (such as aluminium acetate and acetylacetonate) , titanium acetylacetonate, silicon acetylacetonate, possibly other metallic acetylacetonates , and zinc oxide.
In order to reduce the viscosity of the mixture and thereby improve ease of application, and also to increase pot life, the mixture may comprise a molecular architecture modifier or a reaction-retarding catalyst, or a reactive diluent, or any two or all three of these.
The mixture may also contain a reinforcing filler. The filler may be selected from mica, milled exfoliated vermiculite, wollastonite, graphite, calcined china clay, barium sulphate, calcined alumina, fumed silica, and carbon black.
A method according to the invention may also comprise forming at least one hole through said sheet and the coating to provide a passage through the gasket, embossing at least one ridge into said sheet and coating, the ridge extending around said hole, and tempering said sheet by heating to, eg, 420 °C. The hole may be formed by a cutting or a punching operation and the ridge may be formed by pressing between two dies.
There now follow detailed descriptions of four gasket coatings which are illustrative of the invention and of four corresponding methods of forming a gasket having a sealing-enhancing coating which are illustrative of the method aspects of the invention.
All four illustrative coatings provide a micro-sealing coating on a sheet of a multi-layer steel gasket for use between the head and the block of an internal combustion engine. The coating is 30 to 40 micrometres in thickness and extends over at least the sealing region of embossed resilient ridges formed in a steel sheet of the gasket. All four illustrative coatings comprise a silicone resin, specifically polymeric ethoxy functional siloxane, and a siliconate, specifically potassium methyl siliconate, dispersed within said silicone resin. The dispersal of the siliconate is achieved by mixing the resin with a siliconate solution before curing.
In the first illustrative method, the first illustrative coating was formed by forming a spreadable mixture comprising the following components:
200g of silicone resin (100% solids) (Wacker SLM 43220 - an ethoxy-functional dimethylpolysiloxane) ; 60g of aqueous solution of siliconate in potassium hydroxide (42% solids) (Wacker BS15 - it improves the high temperature performance of the coating and may react into the resin backbone on curing) ;
22g of curing agent (polybutyltitanate (100% solids) (Tilcom PBT - it acts as a cross- lmker/catalyst for the resin) ; and
20g of reinforcing filler (mica (Microfme Minerals SX300) ) .
The nominal solids content of the mixture was 88%. The mixture was spread over one surface of a planar sheet of stainless steel of the type conventionally used in multi-layer steel gaskets. The spreading was by means of a K-bar (also known as a Mayer Bar) to a thickness which gave a coating thickness of 30 microns after drying. The mixture was then dried and partly-cured by heating to 200 °C for 10 minutes .
Next m the first illustrative method, holes were formed through the sheet and the first illustrative coating. The holes were formed in the appropriate positions to provide passages through the gasket. Next, the steel sheet and the coating were embossed to give resilient steel ridges coated by the coating, the ridges extended in closed loops around said holes to provide seals around the holes. It was found that the coating remained continuous after the embossing operation.
Finally, m the first illustrative method, the steel sheet and coating were heated to 420 °C for one hour to temper the steel sheet m its embossed form. This did not damage the coating.
When the gasket was subjected to the usual tests, it was found that the first illustrative coating had retained its micro-sealing ability despite the tempering operation. Furthermore, the coating was found to be resistant to oil, water, anti-freeze and f el.
Although the first illustrative coating was found to have excellent fluid and heat resistance, its viscosity was higher than is desirable for some spreading methods. Its viscosity was also unstable and rapidly increased. Thus, the pot life of the mixture was found to be short. Accordingly, a second illustrative coating was formed by a second illustrative method.
In the second illustrative method, the first illustrative method was followed except that the mixture contained:
200g of silicone resin (the same resin mentioned in the first illustrative example) ;
60g of aqueous solution of siliconate in potassium hydroxide (the same solution mentioned in the first illustrative example) ;
25g of curing agent (the same curing agent mentioned in the first illustrative example) ;
50g of a molecular architecture modifier (Wacker TES
40 - see below) ; and
20g of a reaction-retarding catalyst (Wacker catalyst
F100% - aluminium acetylacetonate).
TES 40 is a mixture of tetraethylorthosilicate monomers and oligomers. It is tetra functional and because of its alkoxy functionality can participate in condensation reactions, and can form a tetra functional branch point in the resin network, thus modifying the molecular architecture. It also lowers the viscosity of the mixture prior to coating and extends pot life.
The second illustrative coating was found to have excellent fluid and heat resistance, reduced and more stable viscosity, and increased pot life. However, further improvement was sought by forming a third illustrative coating by a third illustrative method.
In the third illustrative method, the second illustrative method was followed except that the mixture also contained:
50g of a reactive diluent (Dow 3037 - which is a low molecular weight methoxy functional dimethylsiloxane, which has a low viscosity. It reduces the viscosity of the coating material prior to application but, because of its alkoxy-functionality, cross-links into the resin structure on curing) .
The third illustrative coating was found to have excellent fluid and heat resistance, reduced viscosity, and increased pot life.
In the fourth illustrative method, the third illustrative method was followed except that the mixture comprised the following components:
200g of silicone resin as mentioned in the first illustrative example;
20g of a reaction-retarding catalyst (the same catalyst mentioned in the second illustrative example) ;
50g of a molecular architecture modifier (the same mentioned in the second illustrative example) ;
60g of aqueous solution of siliconate in potassium hydroxide (as mentioned in the first illustrative example ;
25g of curing agent (the same mentioned in the first illustrative example) ; and
50g of a reactive dilutent, specifically triethoxyoctylsilane .
The fourth illustrative coating was found to have excellent fluid and heat resistance, reduced viscosity, and increased pot life.
Fillers and pigments can be added to the mixture in a method according to the invention.
Other possible reaction-retarders and diluents are other organosilanes, particularly alkyltrialkoxysilanes, especially methyltriethoxysilanes or octyltriethoxysilane.